1. Field of the Invention
The present invention relates to a method for producing copolymer of ethylene and alpha-olefin.
2. Description of the Related Art
In general, a so-called Ziegler-Natta catalyst, which is composed of a main catalyst component, such as titanium or vanadium compound and a cocatalyst component, such as alkyl aluminum compound, has been used for producing copolymer of ethylene and alpha-olefin. However, Ziegler-Natta catalyst has a high activity for an ethylene homo polymerization, but a poor reactivity for copolymerizing with a high alpha-olefin since it has a non-uniform active catalytic site. Thus, a large dose of alpha-olefin comonomer should be used for producing copolymer of ethylene and alpha-olefin so that there is disadvantage like a catalytic activity is decreased under that condition.
Recently, a so-called metallocene catalyst, which is composed of a metallocene catalyst that is a group IV transition metal in a periodic table, such as titanium, zirconium, hafnium, and the like, and a methlyaluminoxane, as a cocatalyst, has been developed. The metallocene catalyst can produce copolymer of ethylene and alpha-olefin having a uniform distribution of compositions and a narrow distribution of molecular weight as compared to the conventional Ziegler-Natta catalyst since it is a homogeneous catalyst having a single active catalytic site.
Meanwhile, a so-called constrained geometry non-metallocene catalyst, in which a transition metal is connected to a ring shape, has been suggested as a catalyst, which has a high catalytic activity and is capable of producing a polymer having a high molecular weight in copolymerization of ethylene and alpha-olefins under solution polymerization condition. European Patent Nos. 0416815 and 0420436 disclose a constrained geometry non-metallocene catalyst, in which one cyclopentadiene ligand is bonded with amide groups in a ring shape, and European Patent No. 0842939 discloses a constrained geometry non-metallocene catalyst, in which phenol-based ligands, which are electron-donating compounds, are bonded with cyclopentadiene ligands in a ring shape. However, the above constrained geometry non-metallocene catalysts has a significant improved reactivity with a high alpha-olefins due to a decreased sterical hinderance effect of the catalyst itself. However, the above constrained geometry non-metallocene catalysts are very difficult to use commercially because the synthesis of the catalyst is very complicated and the yield of a process of cyclization along the transition metal with ligands during the synthesis of the constrained geometry non-metallocene catalyst is very low.
Meanwhile, U.S. Pat. No. 6,239,238 discloses a non-metallocene catalyst, which is not a constrained geometry catalyst. It can be seen in this patent document that a single-site catalyst, produced using at least one phosphine-imine compound as a ligand, exhibits a high conversion of ethylene in copolymerization of ethylene and alpha-olefin under a high temperature solution polymerization condition at above 140° C. However, the non-metallocene catalysts are not sufficient catalyst for producing copolymer of ethylene and high alpha-olefin since they have very low reactivity with the high alpha-olefin like the metallocene catalyst.